United States Patent
I
[151
3,675,110
Kelley, Jr.
[451
July 4, 1972
[s41 MEANS FOR DAMPING VOLTAGE
[72]
2,730,667
l/l956
Uhlmann ........................... ..32l/11 x
()SCILLATIQNS 1N SOLID.STATE
3,253,138
3732(7) guff
............ ..]. .................... ..32l/45 C
,
,0
reening eta .................... ..321/1 1 X
SWITCHING CIRCUITS
Inventor: Fred W. Kelley, Jr., Media, Pa.
3,532,901 10/1970
3,569,819 3/1971
[73] Assign”: General Electric Company
[22] Filed;
Jam27’1971
Hylten-Cavallius et al.... .... ..32l/ll
Martzloff et al. ...................... ..321/l2
Primary Examiner-William M. Shoop, Jr.
Attorney-J. Wesley Haubner, Albert S. Richardson, Jr.,
Frank L. Neuhauser, Oscar B. Waddell and Joseph B. Forman
[21] Appl. No.: 110,165
,
[57]
[52] US. Cl. ................................ ..32l/l4, 321 /5, 321 /27 R,
321/45 C
[51] 1nt.Cl. ....................................................... ..H02m l/ls
[53] Field of Search ____________ __321/5,11,12,14,27R,45C
[56]
A thyristor is serially joined to an inductor and is shunted by a
snubber circuit comprising a resistor in series with a capacitor.
When subjected to a stepped change in voltage, the series
combination of the inductor and the snubber circuit may
become oscillatory. To prevent the voltage across the thyristor
from undesirably overshooting, a tuned damping circuit is
connected in parallel with the inductor, or alternatively in
parallel with the snubber circuit.
‘(defences Cited
UNITED STATES PATENTS
3,391,327
7/1968
ABSTRACT
Pelly .................................. ..32l/27 R
9 Claims, 3 Drawing Figures
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P'ATENTEDJUL 4 I972
3.675110
SHEET 2 OF 2
ATTORNEY
1
3,675,110
2
MEANS FOR DAMPING VOLTAGE OSCILLATIONS IN
SOLID-STATE SWITCHING CIRCUITS
thyristor can be turned on without a gate pulse by allowing its
anode voltage to increase to a predetermined breakover level
This invention relates to solid-state switching circuits, and,
the dv/dt duty imposed on a thyristor exceeds this critical rate,
there is a danger of premature re?ring and consequent failure
of the inverter mechan'mm.
The snubber circuit is designed to limit the maximum rate of
change of voltage across the valve whenever reverse recovery
current terminates therein or a stepped forward voltage is ap
plied thereto. The snubber capacitor and resistor should be
more particularly, it relates to electric power conversion ap
paratus in which said circuits are used.
There are many known power conversion circuits for
changing the form of electric power from direct current to al
ternating current, from alternating current to unipolarity or
bipolarity direct current, or from polyphase alternating cur
rent of fundamental frequency to single-phase or polyphase al
ternating current of a different frequency. Such apparatus are
popularly referred to as inverters, recti?ers, reversing
recti?ers, cycloconverters, or direct frequency changers (or
multipliers). In each case the conversion is accomplished by
appropriately controlling a plurality of periodically conduct
(V50) or at a rate (dv/dt) in excess of a certain critical value. If
coordinated with the external power circuit inductance to pro
vide a slightly underdarnped series RLC circuit. There are a
number of interrelated and sometimes competing factors that
affect the choice of values of snubber resistance and
capacitance, the principal tradeoffs being dv/dl suppression,
damping, and e?iciency. For example, low resistance is desira
ing, sequentially ?red electric valves that are interconnected
ble for the sake of low losses and to reduce the steepness of
in a bridge con?guration between d-c and a-c temiinals, the
the initial dv/dt imposed on the valve. 0n the other hand,
latter being adapted to be connected to a system of alternating
20 higher resistance helps to limit the initial inrush of current
voltage with which the valve ?rings are synchronized.
contributed to the valve by the discharge of the snubber
in modem practice each valve typically comprises one or
capacitor
when the valve turns on, and it also helps to dampen
more solid-state gate-controlled switching components known
oscillations and thereby limit anode voltage overshoot follow
as semiconductor controlled recti?ers or thyristors. A conven
ing turn off of the valve.
tional thyristor is physically characterized by a body of
One objective of the present invention is to provide im
25
monocrystalline silicon between a pair of main current-carry
proved damping means, useful in combination with thyristors
ing metallic electrodes known as the anode and the cathode,
which are protected by prior art snubber circuits, for con_
respectively. The silicon body may comprise, for example, a
thin, broad area disc-like wafer having four layers of altemate
ly P and N type conductivities, whereby three back-to-back
PN (rectifying) junctions are formed between the main elec
trodes. Usually the wafer is mechanically sealed in an insulat
ing housing and is electrically connected in an external power
trolling anode voltage overshoot with relatively low initial cost
and operating losses.
I have found that in some conversion apparatus the series
RLC circuits in an inactive section of the converter, which cir
cuits are formed by the interaction of the power circuit in
ductance and the snubber resistance and capacitance across
turned-off valves of the inactive section, can become oscillato
circuit by way of its anode and cathode, and the device is
equipped with suitable gating means for initiating conduction 35
ry on the occasion of step changes in the voltage thereacross
between these main electrodes on receipt of a predetermined
due
to switching in another section of the converter. Such
control or trigger signal.
oscillations or ringing can cause overshoot of anode voltage
In operation, each valve of the converter has a nonconduct
ing or blocking statejin which it presents very high impedance
to the ?ow of current, and a conducting or tumed-on state in
on a turned off valve. Anode voltage overshoot is undesirable
because it tends to overstress the snubber resistors, and if the
anode voltage rises too fast _or too high there is a risk of im
which it freely conducts forward current between its main
electrodes. It can be switched abruptly from the former state
proper ?ring of the valve. Accordingly, it is another objective
cyclically recurring instant at which its anode voltage ?rst
becomes positive with respect to cathode, and the magnitude
reactance element, which is part of a snubber circuit for a
thyristor, is serially connected with an inductive reactance ele
ment in a circuit which is subject to relatively abrupt changes
of my invention to provide improved means for damping such
to the latter by applying a trigger signal to its gate when its
oscillations without introducing appreciably higher losses in
main electrodes are forward biased (anode voltage positive
with respect to cathode). This switching or “?ring" action is 45 the apparatus.
A general object of the invention is to provide an improved
ordinarily controlled by associated regulating and control cir
damping circuit which is useful in a variety of solid-state
cuits which supply a train of properly timed and dimensioned
switching circuits that are subject to relatively abrupt changes
gate pulses to the valve. In many converters the moment of
of voltage.
time at which the valve is ?red is expressed in electrical
degrees (known as the “?ring angle”) measured from the 50 In carrying out my invention in one form, a capacitive
of the output voltage of the converter can be varied by retard
of voltage, and oscillation in the circuit is damped by connect
ing or advancing the ?ring angle as desired. Once turned on, a
valve will continue conducting until “forward" current is sub 55 ing across one of the aforesaid reactance elements the series
sequently reduced below a given holding level by the action of
combination
the external circuit in which the valve is conneCted. This tum
off process is referred to generally as “commutation." A
capacitance. The ohmic value of the resistance is selected to
damp oscillations in the circuit on the occasion of the
of a
resistance,
an
inductance,
and
a
variety of line and load voltage commutated converters and
aforesaid abrupt voltage change, and the inductance and
forced commutated inverters are well known in the art.
During commutation, current in a solid-state valve will ac
tually decrease to zero and then reverse directions for a brief
capacitance are tuned to approximately the natural frequency
of the circuit. As a result of this tuning, the resistance can have
a relatively low power rating, and it does not add appreciable
losses to the circuit except during said oscillations.
moment of time until the rectifying junctions inside the
My invention will be better understood and its various ob
thyristors forming that valve have recovered their ability to
withstand reverse voltage, whereupon the valve suddenly 65 jects and advantages 'will be more fully appreciated from the
following description taken in conjunction with the accom
regains its reverse blocking state. Since there is always some
panying drawing in which:
circuit inductance in series with the valve, any sudden cessa
FIG. 1 is a schematic diagram of two three-phase, double
tion of current can lead to undesirable voltage transients
way, six-pulse bridges interconnected to form a reversing
across the valve. To suppress such transients, it is common
practice to shunt the valve with a so-called “snubber" circuit 70 recti?er or cycloconverter type of conversion apparatus with
which my damping means is advantageously used in practice;
comprising a capacitor and a resistor in series.
FIG. 2 is an equivalent circuit diagram of an oscillatory cir
Snubber circuits are also bene?cial in certain inverter appli
cations where a valve is subjected to a very high time rate of
cuit that exists in the FIG. 1 apparatus; and
rise of forward anode voltage at the conclusion of an interval
FIG. 3 is a graph useful for selecting the ohmic value of the
of conduction. Persons skilled in the art are aware that a 75 resistance needed in my damping means to yield a predeter
3
3,675,110
mined overshoot factor, given the damping factor of the
equivalent circuit.
4
current loads, extra bridges can be provided, with their output
terminals respectively coupled to terminals 16d and 16e so as
For purposes of illustrating the presently preferred embodi
to supply additional unidirectional current to the connected
‘ ment of my invention, I have shown in FIG. 1 two duplicate
load. If the input voltages and the valve ?rings of a second for
ward bridge are phase displaced 30 electrical degrees with
respect to the corresponding quantities of the illustrated
bridges which are part of electric power conversion apparatus
that controls the interchange of power between a source 11
and a load 12. These bridges comprise, respectively, the “for
ward section” and the “reverse section” of a whole converter.
bridge, a 12-pulse converter is formed, in which case the in
ductors 13 will perform the function of an interphase reactor.
The conversion apparatus includes control means for
As shown, each bridge comprises a group of six identical elec
tric valves connected in a conventional three-phase, double
way, six-pulse con?guration between three a-c terminals and a
pair of d-c terminals. The same valves could be alternatively
states. Toward this end, an appropriately timed family of dis
arranged in other known con?gurations, such as six-phase,
crete gate pulses is cyclically generated and sequentially ap
periodically switching the respective valves of the forward
bridge from blocking states to forward current conducting
single-way, six-pulse bridges. Preferably both bridges share 15 plied to the respective control electrodes (gates) of the valves
lF-6F, whereby the valves are triggered in numerical
the same a-c terminals a, b, and c which serve as input ter
minals for the illustrated converter, and accordingly the
sequence in synchronism with the alternating voltage of the
source (a conventional phase rotation A-B-C is assumed),
source 11 is connected to these terminals for supplying them
and the ?ow of power through the forward bridge is controlled
with three-phase sinusoidal voltage which alternates at a fun
damental frequency such as 60 hertz. Usually the source is 20 as desired. Firing circuits 17 of suitable design are used for
generating and distributing the requisite gate pulses and for
coupled to the conversion apparatus by means of power trans
former windings (not shown), and it is common practice to
provide overcurrent protective means for suppressing the ?r
ing of the valves, thereby turning off the converter, in
determining their characteristic ?ring angle.
The six valves of the reverse bridge have been identi?ed by
the reference characters 1R through 6R, respectively, and
response to excessively high current in the source connec 25 they are interconnected and arranged similarly to the valves of
the companion forward bridge. The ?ring circuits 18 as
tions.
The d-c terminals of the illustrated bridges serve as the con
sociated with the reverse bridge are similar in function and
verter output terminals. As is shown in FlG. l, the forward and
the reverse bridges of the converter have separate sets of out
operation to the forward ?ring circuits 17.
put terminals which are connected in oppositely poled rela
tionship to the load 12. A ?rst output terminals pF of the for
ward bridge has a positive polarity and is connected directly to
tivating the two ?ring circuits 17 and 18 and for varying their
respective ?ring angles as desired. The average magnitude of
predetermined positive or forward direction when the forward
cyclic commutation process, and each conducting period is
Master control means 19 is provided for alternatively ac
voltage applied to the load circuit can be reduced from max
imum to zero by retarding the ?ring angle from 0° toward 90°.
one side D of the load circuit, while the other output terminal
It will be understood that the master control block 19
nF of the same bridge is negative and is connected via a cur
rent-limiting inductive reactance element 13 to the other side 35 represents suitable control, regulation, and restraint circuitry
to command the proper ?ring circuits 17 or l8 to initiate con
E of the load circuit. On the other hand, the ?rst or positive
duction of the valves of the associated bridge at the proper ?r
output terminal pR of the reverse bridge is connected directly
ing angle to vary the direction and magnitude of current
to the latter side E of the load circuit, and the second or nega
delivered to the load 12 according to a preset or manual pro
tive output terminal nR of this bridge is connected via another
gram. Once turned on, each valve in turn will continue con
current-limiting inductive reactance element 14 to the one
ducting until forward current therein is decreased to zero by a
side B. Thus current can flow through the load circuit in a
immediately followed by an interval of reverse anode-to
bridge is active or in a negative or reverse direction when the
cathode voltage across that valve. It is common practice in the
reverse bridge is active. The load 12 may comprise the arma
ture of a variable speed, reversible d-c motor, or it may in 45 relevant art to provide interlock means 20 so that the opera
tion of the forward ?ring circuits 17 is always inhibited or sup
clude one or more windings of a variable speed, multi-winding
pressed whenever reverse current is being conducted by the
reverse bridge and so that operation of the reverse ?ring cir
cuits 18 is always inhibited or suppressed whenever forward
50 current is being conducted by the forward bridge. More infor
duction by valves in both bridges, the inductors l3 and 14
mation about the function of the interlock means and a useful
which are connected between the bridges will limit short cir
embodiment thereof can be obtained from copending patent
cuit current and thereby prevent serious damage to the valves
application Ser. No. 836,765 ?led on June 26, 1969, for G.R.
while the converter is being turned off by the overcurrent pro
tective means.
55 Lezan and myself and assigned to the General Electric Com
The six valves of the forward bridge have been identi?ed by
pany.
As can be seen in FIG. 1, each valve in the conversion ap
the reference 1F through 6F, respectively. The cathodes of the
paratus is shunted by a snubber circuit comprising a resistor
odd-numbered valves are connected in common to the posi
21 in series with a capacitive reactance element 22. The pur
tive terminal pF of this bridge, and the anodes of the even
numbered valves are connected in common to the negative 60 pose of the snubber circuit was explained in the introductory
portion of this speci?cation, and it is well understood by per
terminal nF. The anode of valve 1F and the cathode of valve
a-c motor.
In normal operation the two bridges of the converter are al'
ternatively active. In the abnormal event of simultaneous con
4F are both connected via the ?rst a-c terminal a of the con
sons skilled in the art. Where thyristors are seriesed to form a
verter to phase A of the alternating voltage source 11. The
valve, each is shunted by an individual snubber circuit, in
which case these circuits will form an R-C bypass network
anode of valve 3F and the cathode of valve 6F are both con
nected via a second a—c terminal b to phase B of the source, 65 which serves the additional bene?cial purpose of forcing
transient and steady-state voltage sharing among the seriesed
and the anode of valve 5F and the cathode of valve 2F are
thyristors.
both connected via the third a-c terminal 0 to the third phase
During operation of the illustrated converter, the respective
V of the source.
valves of a bridge are ?red in numerical sequence as previ
While each of the valves in the forward bridge is illustrated
by a single symbol, in practice it often comprises a plurality of 70 ously explained. Assume that the forward section is active and
that the valves 1F and 2F are conducting load current. In this
separate thyristors connected in series and/or in parallel with
one another and suitably arranged to operate in unison.
Where thyristors are paralleled, it is customary to connect
current balancing inductors in series therewith, and in FIG. 1
such inductors are shown symbolically at 15. For very high
recurrent state of the switching means, the output terminal pF
'. will be energized in accordance with the electrical potential of
the ?rst input temtinal a (i.e., phase A of the source 1 1).
Valve 3F is the next one to operate, and after it has been ?red
5
3,675,110
the output terminal pF is energized in accordance with the
electric potential of the input terminal b (phase B of the
source 11). If the ?ring angle is very retarded, this change in
state of the switching means will occur at a time when the
magnitude of the phase B voltage is appreciably higher than
that of the phase A voltage. As a result, it is accompanied by a
relatively abrupt change of the voltage across the inactive in
ductor 14 in series with the respective even-numbered valves
6
capacitance. Re and C, are equivalent, respectively, to all re
sistors and capacitors in the snubber circuits associated with
the even-numbered valves of the reverse bridge. In practice
the equivalent circuit thus far described may have a damping
in the reverse section of the inverter.
that is relatively low (i.e., less than 0.25).
In particular, the voltage across the series combination of
the inductor 14 and the snubber circuit 21, 22 shunting the
valve 4R, which combination is connected between terminals
pF and a, is subject to stepped increases equal to the dif
The switch 31 in FIG. 2 is periodically operative in a
manner that subjects the series combination of L, and C, to a
ference between the phase B voltage and the phase A voltage
at the time commutation occurs from valve 1F to valve 3F. As
suming that valve 3F was turned on at a retarded ?ring angle,
the voltage increase can be relatively great and commutation
will be completed rapidly, and there is consequently a ten
dency for this series RLC combination to be oscillatory or to
ring and for the voltage across the thyristors in the valve 4R to
rise to an even higher magnitude. Anode voltage overshoots of
as high as 60 percent have been observed in practice. This un
desirably stresses and weakens the resistors 21 in the snubber
circuits. Where each valve comprises two or more thyristors in 25
series and the snubber resistor across one of the thyristors fails
relatively abrupt voltage increase. To damp oscillations in the
series combination on this occasion, the resistance R2 is con
nected across the inductance L2. The degree of damping de
pends on the ohmic value of R2. By selecting an appropriate
value for R2, the overshoot factor of the circuit can be reduced
to an acceptable level. By overshoot factor I mean the max
imum ratio of the output voltage across the thyristor (V0) to
the driving voltage (V,,).
Given the damping factor of the original circuit and the
maximum overshoot that is permissible, persons skilled in the
art will know how to select the proper value of R2. For added
convenience, I have shown in FIG. 3 a family of curves
representing overshoot factor vs. the quantity
(becomes open circuited), all of the voltage will then be im
i E
pressed on that one thyristor which may then turn on by V80
R2
C,
or dv/dt action, whereupon the remaining thyristor is similarly
for a variety of damping factors that are typical of the
?red. As a result of simultaneous conduction by valves in the 30 equivalent circuit shown in FIG. 2. It should be noted that in
forward and reverse bridges, the source 11 is short circuited
plotting these curves I have ignored the equivalent resistance
and the overcurrent responsive protective means will suppress
R, which is trivial if the damping factor is low.
all valve ?rings, thereby shutting down the converter. This
I-Iaving selected the value of R2, I reduce losses in the damp
deenergizes the load 12 at a time that may be harmful to the
ing circuit without appreciable increasing the over-shoot fac
machinery or process being driven thereby.
35 tor by connecting in series therewith the inductance L2 and
In accordance with my invention, the voltage overshoot
capacitance C: which are tuned to the natural frequency of
problem reviewed in the preceding paragraph is solved by
the original circuit. In other words, [.202 equals approximately
shunting either the inductor 14 or the snubber circuit 21, 22
LeCe. As a practical matter, signi?cantly improved results will
with a tuned damping circuit 23 characterized by resistance,
be obtained even if the ratio of these products were greater or
inductance, and capacitance in series with one another. The 40 less than one (e.g., between 1.33 and 0.75 ). The utility of the
resistance has an ohmic value selected to damp oscillations in
tuned damping circuit is improved by keeping C2 as low as
the above-mentioned series combination on the occasion of a
possible, consistent with the following two restraints.
stepped voltage increase, and the inductance and capacitance
are tuned to approximately the natural frequency of the series
2 0.5
combination. Additional information regarding the selection
of parameters will be set forth soon hereinafter. In the illus
trated converter, the tuned damping circuit 23 is shown con~
nected in parallel with inductor 14. There is a duplicate circuit
23 in parallel with the inductor 13 to dampen oscillations in
.
0.25 s
02C?
(R. + 1201+ Ce) s 1
4L2
the forward bridge while the reverse bridge is active. As in 50 The ?rst of these restraints is imposed to ensure that the
dicated in FIG. 1, each of the damping circuits 23 comprises a
tuned damping circuit itself is reasonably well damped. In the
resistor 24 of R2 ohms in series with an inductor 25 of L2 hen
second restraint the fraction is the damping factor of the loop
ries in series with a capacitor 26 of C2 farads. For a better un
comprising the tuned damping circuit in combination with the
derstanding of the criteria for selecting these parameters, 55 output circuit R,C,; the economics of my invention becomes
FIGS. 2 and 3 have been included in the drawing, and they will
unfavorable if this damping factor is higher than 1.
now be described.
As was previously mentioned, my tuned damping circuit can
In FIG. 2 an equivalent circuit of the FIG. 1 converter is
be connected alternatively in parallel with the snubber circuit.
shown. The switch 31 is equivalent to the controllable
This possibility, which is illustrated by broken lines in FIG. 2,
switching means or valves of the forward section of the con 60 may be advantageous in forced commutation inverters or for
verter, and the closure of this switch corresponds, for exam
any case where the main inductance Le is inaccessible.
pie, to the ?ring of valve 3F and rapid commutation from
When connected in this manner, my invention will bene?
phase A to phase B of the source 11 in FIG. 1. The battery 32
cially augment the conventional damping function of the
in FIG. 2 is a source of voltage (V,,) equal to the voltage
snubber capacitor on the occasion of turning o?‘ the thyristor
between phase B and phase A when the switch 31 is closed. 65 33.
The equivalent inductance Le equals the sum of the in
While I have shown and described a preferred form of the
ductances of the current limiting reactive element 14 and the
invention by way of illustration, many modi?cations will
balancing reactors 15. (As a first approximation, I... can be
probably occur to those skilled in the art. I therefore contem
considered the same as L,, which is the inductance of the cur
plate by the claims which conclude this speci?cation to cover
rent-limiting inductor and is usually much larger than that of 70 all such modi?cations as fall within the true spirit and scope of
the invention.
the balancing reactors.) The thyristor 33 represents a turned
off valve (e.g., 4R) in the reverse section of the converter, and
What I claim as new and desire to secure by Letters Patent
V0 is the “output” voltage thereacross. Capacitance of Ce
of the United States is:
farads is connected in parallel with thyristor 33, and resistance
1. In an electric power circuit comprising a thyristor, a
of R, ohms is connected across the thyristor in series with this 75 capacitive reactance element connected in parallel with said
3,675,110
8
7
said resistance having an ohmic value selected to damp
oscillations in said series combination on the occasion of
thyristor, an inductive reactance element connected in series
with the parallel combination of said thyristor and said capaci
tive reactance element, a plurality of input terminals adapted
said abrupt voltage change, and said inductance and said
capacitance being tuned to approximately the natural
frequency of said series combination.
to be connected to a source of voltage, and means including
controllable switching means for electrically interconnecting
said input terminals and the series combination of said capaci
5. The converter of claim 4 in which means is provided for
connecting said ?rst output terminal directly to one side of an
electric power load circuit and for connecting said second out
put terminal via a current limiting inductor to the other side of
the load circuit.
6. The converter of claim 4 in which said input terminals are
adapted to be connected to a source of 3-phase alternating
tive reactance element and said inductive reactance element,
said switching means being periodically operative in a manner
that subjects said series combination to a relatively abrupt
change of voltage, the improvement comprising:
a tuned damping circuit connected in parallel with a
predetermined one of said reactance elements, said
voltage, means is provided for directly connecting said ?rst
damping circuit being characterized by resistance, in
output terminal to one side of an electric power load circuit
ductance, and capacitance in series with one another,
said resistance having an ohmic value selected to damp 15 and for connecting said second output terminal to the other
side of the load circuit, and said controllable switching means
oscillations in said series combination on the occasion of
comprises a group of six periodically conducting electric
said abrupt voltage change, and said inductance and said
valves connected in a ?rst bridge con?guration between said
capacitance being tuned to approximately the natural
frequency of said series combination.
input and output terminals, said valves being cyclically ?red,
20 when the ?rst bridge is active, in a predetermined sequence so
2. The circuit of claim 1 in which said predetermined one
as to supply unidirectional current to the connected load cir
reactance element is said inductive reactance element.
cuit.
3. The circuit of claim 1 in which a resistor is connected
7. The subject matter of claim 6 wherein the converter com
across said thyristor in series with said capacitive reactance
prises two groups of valves forming ?rst and second duplicate
element.
25 bridges which share the same input terminals but have
4. In an electric power converter comprising a thyristor, a
separate sets of output terminals, said bridges being alterna
capacitive reactance element connected in parallel with said
tively active and their respective sets of output terminals being
thyristor, an inductive reactance element connected in series
connected in oppositely poled relationship to said load circuit
with the parallel combination of said thyristor and said capaci
so that current can ?ow through said load circuit in a
tive reactance element, a plurality of input terminals adapted 30 predetermined forward direction when the ?rst bridge is ac
to be connected to a source of voltage, a set of at least ?rst and
tive or in a reverse direction when said second bridge is active,
second output terminals, means including controllable
each valve of said second bridge comprising at least one
thyristor which is shunted by a capacitive reactance element;
switching means for electrically interconnecting said input
and said inductive reactance element is connected between
and output terminals, said switching means having one recur
rent state in which said ?rst output terminal is energized in ac 35 the second output terminal of said second bridge and the ?rst
output terminal of said ?rst bridge. '
cordance with the electrical potential of a ?rst one of said
8. The converter of claim 7 in which said predetermined
input terminals and being periodically operative to another
reactance element is said inductive reactance element.
state in which said ?rst output terminal is energized in ac
9. The converter of claim 8 in which the ?rst output ter
cordance with the electrical potential of another one of said
minal of said second bridge is directly connected to said other
input terminals, the series combination of said capacitive
side of the load circuit, each valve of said ?rst bridge com
reactance element and said inductive reactance element being
prises at least one thyristor which is shunted by a capacitive
connected between said ?rst output terminal and said ?rst
reactance element, an additional inductive reactance element
input terminal, whereby upon operation of said switching
is connected between the second output terminal of said ?rst
45 bridge and the ?rst output terminal of said second bridge, and
a relatively abrupt change, the improvement comprising:
said additional inductive reactance element is shunted by
a tuned damping circuit connected in parallel with a
another tuned damping circuit which is a duplicate of the
predetermined one of said reactance elements, said
means the voltage across said series combination is subject to
tuned damping circuit speci?ed in claim 1.
damping circuit being characterized by resistance, in
ll‘
ductance, and capacitance in series with one another,
50
55
60
65
75
*
*
*
*